Of the ions involved in the intricate workings of the heart, calcium is considered perhaps the most important. It is crucial to the very process that enables the chambers of the heart to contract and relax, a process called excitation-contraction coupling. It is important to understand in quantitative detail exactly how calcium is moved around the various organelles of the myocyte in order to bring about excitation-contraction coupling if we are to understand the basic physiology of heart function. Furthermore, spatial microdomains within the cell are important in localizing the molecular players that orchestrate cardiac function.

Cardiac excitation–contraction coupling

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Physiology of Microglia

Spontaneous local increases in the concentration of intracellular calcium, called "calcium sparks," were detected in quiescent rat heart cells with a laser scanning confocal microscope and the fluorescent calcium indicator fluo-3 Estimates of calcium flux associated with the sparks suggest that calcium sparks result from spontaneous openings of single sarcoplasmic reticulum (SR) calcium-release channels, a finding supported by ryanodine-dependent changes of spark kinetics At resting intracellular calcium concentrations, these SR calcium-release channels had a low rate of opening (approximately 00001 per second) An increase in the calcium content of the SR, however, was associated with a fourfold increase in opening rate and resulted in some sparks triggering propagating waves of increased intracellular calcium concentration The calcium spark is the consequence of elementary events underlying excitation-contraction coupling and provides an explanation for both spontaneous and triggered changes in the intracellular calcium concentration in the mammalian heart

https://science.sciencemag.org/content/sci/262/5134/740.full.pdf

Calcium sparks: elementary events underlying excitation-contraction coupling in heart muscle

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

https://www.physiology.org/doi/pdf/10.1152/physrev.1999.79.3.763

Sodium/Calcium Exchange: Its Physiological Implications

https://www.cell.com/cell/pdf/0092-8674(95)90359-3.pdf

SCN5A mutations associated with an inherited cardiac arrhythmia, long QT syndrome

The photoprotein aequorin was micro-injected into several cells in a sheep Purkinje fibre. The intracellular Ca concentration [( Ca2+]i) was measured from the resulting light emission. Inhibition of the Na-K pump with strophanthidin resulted in the development of tonic tension which increased on depolarization. This increase was accompanied by an increase of aequorin light. Increasing external Ca concentration [( Ca2+]o) or the magnitude of the depolarization increased both light and tension. If the depolarizing pulse was maintained for several minutes then both tonic tension and aequorin light slowly decayed. The relationship between tension and light was unaffected during this decay. On repolarization the light decayed to below the level before the depolarization before slowly increasing. During this period a test depolarization produced increases of aequorin light and tension which were smaller than control. The application of ryanodine (1-10 microM) abolished all components of tension other than the tonic component. Under these conditions the time course of the increase of tonic tension and aequorin light on depolarization was sufficiently slow to be measured. In most (five out of six) experiments the relationship between light and tension during this development of tonic tension was found to be similar to that during the subsequent spontaneous decay. However, in one experiment the decay of force was greater than could be accounted for by the fall of [Ca2+]i. It is concluded that most of the spontaneous relaxation of tonic tension can be attributed to a fall of [Ca2+]i rather than to other explanations such as an intracellular acidification or increase of inorganic phosphate concentration.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1986.sp016116

Measurement of intracellular calcium during the development and relaxation of tonic tension in sheep Purkinje fibres.

Two centuries of excitation-contraction coupling

Rationale: Phosphodiesterase-5 (PDE5) inhibition reduces the occurrence of ventricular arrhythmias following myocardial ischemia. However, the mechanisms of the anti-arrhythmic effects of PDE5 inhibition are unknown. Diastolic calcium (Ca 2+ ) waves lead to arrhythmias by inducing delayed after-depolarizations. Ca 2+ waves are initiated when sarcoplasmic reticulum (SR) Ca2+ content reaches a threshold level and the SR releases Ca 2+ spontaneously and generates a depolarizing inward sodium-calcium exchange (NCX) current. Objective: To determine the effects of PDE5 inhibition on the propensity for ventricular arrhythmias in a pro-arrhythmic large animal model and establish the role of alterations of intracellular Ca 2+ cycling / SR Ca 2+ content. Methods and Results: Arrhythmia burden, monophasic action potentials and beat-to-beat variability of repolarization were measured in a sheep model using the I Kr inhibitor dofetilide to induce QT prolongation and arrhythmia. Ca 2+ transients, Ca 2+ waves and SR Ca 2+ content were measured in isolated ventricular myocytes. PDE5 inhibition was achieved using acute application of sildenafil and protein kinase G (PKG) was inhibited with KT5823. PDE5 inhibition reduced beat-to-beat variability of repolarization and suppressed after-depolarizations, premature ventricular complexes, and torsade de pointes in vivo. In single cells, dofetilide-induced DADs and triggered action potentials were suppressed by PDE5 inhibition. PDE5 inhibition decreased Ca 2+ wave frequency in all cells and abolished waves in 12/22 cells. A decrease in SR Ca 2+ uptake, increased trans-sarcolemmal Ca 2+ efflux and reduced trans-sarcolemmal Ca 2+ influx led to a reduction of SR Ca 2+ content, and Ca 2+ wave abolition. These effects were dependent on PKG activation. Conclusions: PDE5 inhibition acutely suppresses triggered ventricular arrhythmias in vivo and cellular data suggests this occurs via suppression of cellular Ca 2+ waves. These novel anti-arrhythmic properties of PDE5 inhibition are mediated by a reduction of SR Ca 2+ content and are PKG-dependent.

https://www.ahajournals.org/doi/pdf/10.1161/CIRCRESAHA.121.318473

PDE5 Inhibition Suppresses Ventricular Arrhythmias by Reducing SR Ca2+ Content.

Activity dependent changes in mammalian ventricular muscle [proceedings].

Yamamoto et al. [1] have shown in their recent article that the expression of cardiac ryanodine receptors is decreased in dogs subjected to heart failure induced by rapid-pacing. This agrees with previous work showing that in some models of failure down regulation of the RyR is observed [2,3]. As well as a decrease in the number of RyRs, it has also been suggested that there may be a decrease in the functional coupling between Ca entry into the cell and the opening of the RyR [4,5]. The purpose of this letter is to question the idea that changes in the expression or properties of the RyR are involved in changes of contractility observed in cardiac hypertrophy or failure. The argument below is based on both experimental and theoretical considerations [6].

We have examined the effects of manoeuvres which alter the open probability of the RyR. For example caffeine …

David A. Eisner

Papers

Measurement of intracellular calcium during the development and relaxation of tonic tension in sheep Purkinje fibres.

Two centuries of excitation-contraction coupling

PDE5 Inhibition Suppresses Ventricular Arrhythmias by Reducing SR Ca2+ Content.

Activity dependent changes in mammalian ventricular muscle [proceedings].

Can changes of ryanodine receptor expression affect cardiac contractility